1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
A wireless network typically consists of a plethora of network elements that collectively provide data and voice communications for nomadic users through a macro-cellular environment. For example, a conventional Evolution, Data-Optimized (EvDO) cellular network includes one or more base transceiver stations (BTS), radio network controllers (RNC), packet data switch nodes (PDSN), foreign agents (FA) and home agents (HA). In the EvDO network, the BTS provides radio access to nomadic access terminals, the RNC provides for radio channel mobility, the PDSN maintains an Internet Protocol-Point to Point Protocol (IP-PPP) tunnel between the PDSN and the access terminal. Lastly the HA and FA provide macro-mobility functionality for nomadic access terminals. In a typical cellular network, there are many BTSs, relatively fewer RNCs, PDSNs and FAs, and still fewer HAs.
Data transmission between the PDSN and the mobile terminal is byte oriented, while data transmission between the PDSN and the HA is based on transmitting whole IP packets. To transmit packets, such as IP version 4 (IPv4) packets, between the home network (i.e. HA) and the PDSN, the HA tunnels the IP packets to the FA by way of an IP-in-IP tunnel, e.g., by extending the IP packets with an additional header indicating an address of the FA. Once a packet is received by the FA, the FA transmits the packet to the PDSN for further delivery to the access terminal over a PPP tunnel between the PDSN and the access terminal. The PDSN and FA are typically co-located to simplify delivery of IP packets to the appropriate PPP tunnels. Conversely, packets transmitted by the access terminal are forwarded to the PDSN/FA over a PPP tunnel, which may then transmit the packets via an IP-in-IP tunnel to the home agent for eventual transmission onto an external network, such as an Internet. Thus, packets transmitted on either the uplink or downlink will be transmitted to the home agent before reaching their final destination.
A substantial fraction of the calls within a wireless communication system are generally thought to be local calls. This means that a voice call originating within a particular geographic area is also likely to terminate in the same geographic area. However, voice calls that are handled using various Internet protocols need to take a circuitous route that may take individual packets far from their point of origin before returning to nearly the same location. For example, the voice samples for Voice over Internet Protocol (VoIP) calls are encoded in RTP/UDP/IP packets and are routed between source and destination through standard IP routing mechanisms. Thus, the packets transmitted between source and destination access terminals are still routed back by way of a BTS, RNC, PDSN and foreign agent to the home agent even if the source and/or destination access terminals have roamed away from their home networks. This is a particularly inefficient way to route voice traffic given that it is assumed a fraction of voice calls in a cellular network are local and that a packet typically travels through a relatively large network (i.e., distance and number of network elements) before it reaches its home agent.